1. Field of the invention
The invention relates to a process for the preparation of dichlorophene (5,5'-dichloro-2,2'-dihydroxy-diphenylmethane) from p-chlorophenol and formaldehyde in sulphuric acid.
2. Discussion of Prior Art
Because of its microbicidal action, dichlorophene is used as an industrial preservative, for example for industrial woollen goods, heavy fabrics and cooling lubricants. In hygiene and medicine, it is used as an antiseptic and anthelmintic (British Pharmacopeia 1973, page 157).
In selecting a process of preparation, it is necessary to take into account the purity standards relating to the particular end use of the dichlorophene. For use as an industrial preservative, a residual content of p-chlorophenol of at most 0.8% is desired. Furthermore, it is advantageous if concentrated, residue-free solutions in aqueous sodium hydroxide solution or in an alcohol, such as butanol can be prepared. Inadequate solubility is in most cases due to an excessive content of products of higher molecular weight from side reactions. Where the preservation of non-coloured products such as, for example, paper is concerned, a low intrinsic colour of the dichlorophene is desirable in addition.
For the hygiene field and the veterinary medicine field, a p-chlorophenol content of less than 0.1% is required for toxicological reasons (British Pharmacopoeia 1973, page 156 to 157, British Veterinary Codex 1965, page 134).
To assess the purity of the dichlorophene, and assess the p-chlorophenol content, it is possible to employ not only the solubility in aqueous alkali or in alcohols, but also the melting point. Chemically pure dichlorophene melts at 177.degree. to 178.degree. C.
The melting point is lowered particularly by the presence of p-chlorophenol and other impurities of a low molecular weight. German Reich Patent No. 530,219 has disclosed the manufacture of dichlorophene by reacting p-chlorophenol with formaldehyde in the molar ratio of 2 : 0.96 to 1.09 in 50% strength sulphuric acid at 50.degree. to 65.degree. C. It is true that no data regarding the purity of the dichlorophene obtained are given in German Reich Patent No. 530,219, but the dichlorophene prepared in accordance with the instructions of this patent specification is dark in colour and contains unconverted p-chlorophenol and more highly condensed by-products which are detrimental to the solubility. A crude product obtained in this way is therefore unsuitable for industrial use. The unconverted p-chlorophenol can be removed by steam distillation, whereby the intrinsic colour of the crude product becomes even deeper. The dark-coloured and the sparingly soluble more highly condensed reaction products can be removed afterwards by clarification of an aqueous solution of the sodium salt of dichlorophene and reprecipitation of the purified dichlorophene with mineral acids.
The disadvantages of the process described above are avoided, according to U.S. Pat. No. 2,334,408, by carrying out the condensation in methanol as the solvent, with concentrated sulphuric acid as the catalyst, at temperatures of -10.degree. to 0.degree. C. According to British Pat. No. 1,208,325, an improvement in this process is achieved by a continuous operation under similar reaction conditions, the amount of formaldehyde required for the condensation being supplied stepwise to the reaction mixture over the course of a three-step kettle cascade. The continuous method in this case essentially serves to give improved heat control of the process of preparation. The dichlorophene can be precipitated from the reaction mixture by dilution with water, in a purity which permits its use as an industrial preservative. The melting point of the dichlorophene prepared in this way is 165.degree. to 167.degree. C; the yields, based on the p-chlorophenol employed, are 90 to 93% of theory.
Compared to the method of German Reich Patent No. 530,219, the latter processes admittedly give a dichlorophene which is usable industrially without additional purification steps, but the use of solvents leads to additional problems in working up. Furthermore, the methanol/sulphuric acid mixture is diluted with water, as a result of the precipitation of the dichlorophene, to such an extent that a direct reuse for condensation is impossible.
It is a feature common to all the processes described above that the total amount of p-chlorophenol is added at the start of the reaction and the formaldehyde is only employed in a molar ratio of up to 0.505, relative to the p-chlorophenol. The large excess of p-chlorophenol established at least in the main stage of the reaction is considered essential in order to restrict excessive formation of higher molecular byproducts (British patent specification No. 1,208,325).